Originally posted by 9ine_Lives
Originally posted by iori_komei
prophecy,
WolfOfWar when people moved north to colder climates white skin developed to absorb more sunlight...
If they evolved to adapt to colder surroundings...wouldent it be logical to do so as darker pigmentation? it absorbs more sunlight and heat energy...
It has nothing to do with heat. It has to do with light. Ultraviolet light, to be precise.
If our planet didn't have an atmosphere, the amount of UV energy it received from the sun would be enough to fry anything living on the surface. But
the atmosphere absorbs most of it, so only a relatively small amount reaches the ground. Even this residue is enough to cause plenty of trouble. It
causes the mutations in human skin cells that give rise to cancer, for example.
Near the Equator, incoming UV light strikes the atmosphere at a very steep angle, anything up to 90 degrees. So it has relatively less atmosphere to
pass through compared with UV light that strikes the atmosphere near, say, the Arctic Circle. The higher the latitude, the more atmosphere the UV has
to pass through before it hits the earth's surface, therefore the less energetic it will be when if finally strikes the ground.
What this means is that our planet receives a stronger dose of UV around the Equator than it does nearer the Poles.
Accordingly, tropical life-forms have evolved defences against ultraviolet radiation that temperate and arctic life-forms don't need.
One of them (evolved in humans) is melanin. Melanin protects us against the damage caused by high-energy UV radiation.
Now, as human beings migrated northward out of Africa, the selective pressure to maintain dark skin (a high melanin count) lessened, because the
thicker atmospheric blanket between them and the sun made incoming UV much less of a problem.
On the other hand, since there's less sunlight all round in temperate latitudes, the mechanism by which the body synthesizes vitamin D from sunlight
has to become more efficient.
Melanin interferes with this mechanism. So a
negative selection pressure evolved with respect to melanin.
And folk started turning white.
Yes, I know, I know, another far-fetched theory from those confounded scientists. So much easier to believe in Atlanteans, or Bible stories, no?
Check the pic by clicking on it. If it looks small then keep
clicking on it to zoom it on. 
